This invention relates to techniques for assisting patients to breathe, and, more particularly, to an approach for synchronizing an external ventilator to the spontaneous breathing of the patient.
Undersized and premature infants, as well as full-term infants, often have difficulty breathing. The inability to breath regularly results in insufficient aeration of the blood of the baby, in turn leading to or exacerbating other problems. As with breathing disorders in older persons, the infant may devote such a large amount of its energy to breathing that it cannot overcome infections and disorders.
It is sometimes observed that the infant's condition can be remarkably improved simply by ensuring a regular air supply that permits its energy to be directed elsewhere. Most undersized and premature babies are therefore placed onto a program of breathing assistance with a device termed a "ventilator". In simplest terms, the ventilator either forces pressurized gas into the lungs (e.g., a positive-pressure ventilator) or expands the chest cavity to draw gas into the lungs (e.g., a negative-pressure ventilator such as an iron lung) under a selectable schedule of gas composition, pressure, and flow pattern.
While negative-pressure ventilators enjoyed a degree of popularity in the past, their use has been largely replaced by positive-pressure ventilators. The positive-pressure ventilator is a mechanical device external to the patient, which creates an external pressure and thereby forces gas into the patient's lungs through a tube, termed the airway. The gas may be air, pure oxygen, air enriched with additional oxygen, or some other oxygen-containing mixture.
The ventilator can operate in several different modes. In one, the ventilator introduces gas into the lungs of the patient on a regular basis, without any effort by the patient. (As used herein, a "breath" is an inhalation of gas made by the patient with its own efforts, while "ventilation" is the introduction of gas into the lungs that can occur either due to a breath, or in the absence of a breath by the operation of the ventilator.) In another mode of operation, the patient does breathe, but the ventilator supplies additional gas to that which the patient could itself bring into its lungs, or the ventilator supplies intermediate ventilations in addition to those made by the patient with its own efforts.
In another mode of operation, the operation of the ventilator is synchronized to the breathing of the patient, so that the ventilator is not forcing gas into the lungs at the same time that the patient is attempting to breathe out. The present invention is concerned with the control of the ventilator so that it is synchronized with the breathing efforts of the patient.
The present invention finds its greatest application to infant patients because of their special requirements, but is also useful for other patients with specialized breathing problems. The breathing of undersized and premature infants differs from that of adults in at least two important respects. Infants generally breathe much faster than do adults. Infants typically breathe at a rate of 30 or more breaths per minute and often breathe at irregular intervals, while adults at rest typically breathe about 15 times per minute in regular intervals. The inspiration time required to draw the breath into the lungs is typically about 200-400 milliseconds (0.2-0.4 seconds) for an infant. Second, the lungs of the infant are much smaller than those of the adult, so that the volume of gas inhaled and exhaled during a breath, and the pressure within the lungs, are smaller than those of the adult.
Synchronization of the ventilator to the breathing of infants is therefore much more difficult than similar synchronization to adults, because it is difficult to determine when the baby breathes and because the reaction of the ventilator must be very fast once the initiation of a breath is detected. Studies have indicated that the ventilator must start its ventilation within some short time interval, typically about 60 milliseconds or less, after the baby begins its voluntary breath, or the ventilator may provide little assistance or may actually end up working against the infant's own efforts.
Adult ventilators are typically synchronized with the adult's own breathing by placing a proximal pressure sensor in the airway of the ventilator just outside the patient. A sufficiently lower pressure in the airway, relative to the positive end expiratory pressure (PEEP), is then interpreted as a breath. Because of the relatively slow, regular breathing and relatively large pressure changes, the initiation of the breath is readily determined.
The use of such a technique for an infant is not practicable for the reasons indicated. In another approach under study for infants, a sensor that measures the movement of the abdomen of the infant is used to trigger ventilator operation when the movement exceeds a selected value. Studies with this approach have obtained triggering within about 110-120 milliseconds of the spontaneous initiation of the breath, but have not attained initiation within the desired 60 milliseconds. With triggering at 110-120 milliseconds, the inhalation of gas by the baby is about 2/3 complete before the ventilator begins to provide any assistance. It is therefore now common practice not to use a synchronized breathing mode for ventilation of undersized and premature infants, because no generally satisfactory synchronization approach exists.
There exists a need for a better approach to the synchronization of a ventilator to a patient's own breathing, particularly for infant patients. Such an improved approach would significantly improve the potential for respiratory care of the patients. The present invention fulfills this need, and further provides related advantages.